Periodical cicadas typically disappear about four to six weeks after they first start crawling out of the ground. The adults emerge, mate, lay eggs, and die in a compressed burst of activity, and then they are gone for another 13 or 17 years depending on the species. Annual cicadas, which belong to different genera entirely, show up every summer and tend to fade by early autumn. But the spectacle most people associate with the question “when will the cicadas be gone?” involves periodical cicadas, whose synchronized mass emergences are among the most dramatic events in North American entomology.
What Happens During Those Few Weeks Above Ground
After spending over a decade underground, periodical cicadas tunnel to the surface once soil temperatures at about eight inches deep reach roughly 64°F (18°C). In most of the eastern United States, that happens in late April or May. The nymphs climb the nearest vertical surface, split their exoskeletons, and emerge as pale, soft-bodied adults. Within a few hours their wings expand and their bodies darken.
Males begin calling within days, producing the loud, pulsing chorus that defines a cicada emergence. Each of the three species groups of 17-year periodical cicadas produces a distinct call, and females use those calls to identify appropriate mates. The sound environment during a mass emergence is extraordinarily loud and complex, and research on their acoustic systems shows that some species have evolved sharply tuned resonating structures and unusually low-frequency songs, likely adaptations for communicating clearly to females at close range amid the din of millions of overlapping singers.1Biological Journal of the Linnean Society. Acoustic adaptations of periodical cicadas (Hemiptera: Magicicada)
After mating, females use a saw-like ovipositor to cut slits in pencil-sized tree branches and deposit eggs inside the wood. Each female may lay several hundred eggs across multiple branches. The egg-laying causes visible damage, a phenomenon called “flagging,” where the tips of branches turn brown and droop. Within about six weeks of the first emergences, virtually all the adults have died. The carcasses litter the ground, the noise stops, and the only remaining trace is the egg-laden branch tips overhead.
The Long Underground Phase
The above-ground spectacle is only the final chapter of a life that spans either 13 or 17 years, almost all of it spent underground. Tiny first-instar nymphs hatch from eggs in the tree branches roughly six to ten weeks after being laid, then drop to the ground and burrow into the soil. Once underground, they attach to tree roots and begin feeding.
Contrary to what many people assume, cicada nymphs do not feed on sap in the way aphids do. Studies examining root tissue under a microscope found that periodical cicada nymphs of all developmental stages feed exclusively on xylem, the tissue that carries water and dissolved minerals upward from roots. No feeding traces were found in phloem, the sugar-rich tissue that most sap-feeding insects target.2Ecological Entomology. Xylem feeding by periodical cicada nymphs on tree roots Xylem fluid is nutritionally dilute, which helps explain why development takes so long. The nymphs pass through five growth stages over their 13 or 17 years, slowly increasing in size while feeding on this low-calorie diet deep in the soil.
How Nymphs “Know” When to Emerge
One of the longest-standing mysteries about periodical cicadas is how an insect underground, attached to a root in darkness, manages to count years and synchronize with millions of others across a wide geographic area. Recent field research has tested what scientists call the “four-year gate” hypothesis. The idea is that nymphs reach decision points every four years where they assess their own body weight. If a nymph has reached a critical threshold, it commits to emerging the following spring. If not, it waits another four years.
A study examining nymphs of different ages at multiple field sites found strong support for this model. Almost all 16-year-old nymphs showed the telltale red eye color that signals a commitment to emerge, and they had body weights large enough to have crossed the critical threshold. A small but real fraction of 12-year-old nymphs with unusually large body weights also showed the red-eye switch, suggesting they had hit the threshold a full cycle early.3Proceedings of the Royal Society B: Biological Sciences. When and how do 17-year periodical cicada nymphs decide to emerge? A field test of the 4-year-gate hypothesis This mechanism neatly explains both the rigid synchrony of most individuals and the occasional “straggler” emergences that happen four years early or four years late.
Why 13 and 17 Years, Specifically
Both 13 and 17 are prime numbers, and this is probably not a coincidence. The leading evolutionary explanation holds that prime-numbered cycles minimize overlap with potential predators or parasites that might operate on shorter, regular cycles. A predator with a two-year boom-bust cycle would align with a 12-year cicada every 12 years, but would align with a 13-year cicada only every 26 years. Prime cycles are harder for any regularly cycling enemy to exploit.
But how did these specific cycle lengths arise in the first place? Modeling work points to glacial periods as the trigger. During cold climate phases, longer growing seasons were needed for nymphs to reach maturity, stretching juvenile periods to extreme lengths. Those long development times made adult densities very low in any given year, which created strong evolutionary pressure to synchronize emergence so adults could actually find mates.4PubMed Central. Evolution of periodicity in periodical cicadas Once synchronized emergence was established, selection for prime-numbered cycles could have further refined the system.
Genetic studies show that the split between 13-year and 17-year species is surprisingly shallow. Within each of the three main species groups, the 13-year and 17-year forms are very closely related, with small genetic divergences between them despite major differences in life-cycle length.5PubMed Central. Mitochondrial Genomics Reveals Shared Phylogeographic Patterns and Demographic History among Three Periodical Cicada Species Groups This pattern suggests that the switch between cycle lengths has happened independently in each species group, possibly more than once, and that the difference between 13 and 17 years may come down to differences in nymphal growth rate rather than deep evolutionary divergence.6Ecological Research. Life‐cycle control of 13‐ and 17‐year periodical cicadas: A hypothesis and its implication in the evolutionary process
Safety in Overwhelming Numbers
The sheer density of a periodical cicada emergence is not incidental to their survival strategy. It is the strategy. Birds are the primary predators of adult cicadas, and the fundamental idea behind synchronized mass emergence is that predators simply cannot eat them all. This is called predator satiation.
Field studies of 13-year cicada emergences tracked exactly how this plays out over the course of a season. Early-emerging cicadas in May were heavily consumed by birds. But within days, avian predators became full. At low cicada densities, birds ate a substantial share of the local population, but when cicada densities exceeded roughly 24,000 individuals per hectare, birds consumed only a tiny fraction of the available cicadas. The predators stayed satiated for several weeks. By the time birds needed more food to feed their growing nestlings in June, cicada density had already dropped naturally as the adults reached the end of their lives.7Ecology. Emergence of 13‐Yr Periodical Cicadas (Cicadidae: Magicicada): Phenology, Mortality, and Predators Satiation
Further research confirmed that predator numbers did not increase in response to the cicada emergence. Bird populations were essentially fixed on a timescale of weeks, so as cicada density went up, each individual cicada’s risk of being eaten went down.8Ecology. Increased Reproductive Success at High Densities and Predator Satiation For Periodical Cicadas This is why stragglers that emerge a year or two off-schedule are at a severe disadvantage. Without the protection of the swarm, they get picked off easily.
A Fungal Hijacker
Not all cicadas that emerge will die of old age or predation. A specialized fungal pathogen called Massospora cicadina infects some periodical cicadas in a particularly unsettling way. The fungus consumes the cicada’s abdomen from the inside, replacing the rear segments with a mass of fungal spores while the cicada continues to fly and behave somewhat normally.
Research on infected males revealed something stranger still. Males carrying early-stage infections began performing wing-flick signals that are normally produced only by sexually receptive females. These fake female signals attracted other males, who attempted to mate with the infected individual, picking up fungal spores in the process. Males with later-stage infections, which produce a different type of resting spore intended to persist in the soil until the next emergence cycle, did not produce these signals.9PubMed Central. A specialized fungal parasite (Massospora cicadina) hijacks the sexual signals of periodical cicadas (Hemiptera: Cicadidae: Magicicada) The fungus effectively turns its host into a transmission vehicle, manipulating cicada behavior to serve its own reproductive interests. It is one of the more remarkable examples of parasite manipulation of host behavior known in insects.
What Billions of Dead Cicadas Do to the Ecosystem
When the adults die, their bodies deliver a massive pulse of nutrients to the forest floor. Billions of cicadas decomposing at once release nitrogen and other elements into the soil. Researchers studying this effect found that adding cicada bodies to the soil around trees increased plant-available nutrient levels and boosted populations of soil invertebrates that feed on microbes involved in decomposition. However, in a four-year follow-up, the nutrient pulse did not translate into measurable improvements in tree growth or performance.10PubMed Central. Acute resource pulses from periodical cicadas propagate to belowground food webs but do not affect tree performance The ecological effects are real but appear to be concentrated in the soil food web rather than working their way up to visible changes in the trees themselves.
Annual Cicadas Follow a Different Schedule
If you hear cicadas buzzing every summer, those are not periodical cicadas. Annual cicadas, sometimes called dog-day cicadas, belong to entirely different genera and emerge every year. Individual annual cicadas also spend multiple years underground as nymphs, but because their generations overlap, some adults appear every summer without fail. They tend to emerge later in the season than periodical cicadas, typically from July through September, and are generally larger and green or brown rather than the distinctive black-bodied, red-eyed appearance of periodical species. Annual cicadas are found across most of the world, while periodical cicadas exist only in the eastern United States.
Could Climate Change Shift the Schedule
Warmer temperatures could speed up nymphal development, and there is evidence this has already happened. Periodical cicada broods occasionally produce “accelerations,” where a subset of individuals emerges four years ahead of schedule. Researchers have hypothesized that these four-year accelerations occur during warmer periods when growing seasons are longer and nymphs can reach their critical body weight earlier than expected.11PubMed Central. The periodical cicada four-year acceleration hypothesis revisited and the polyphyletic nature of Brood V, including an updated crowd-source enhanced map If enough individuals accelerate together, they can establish a new brood on a shifted schedule. Some existing broods may have originated this way.
Range shifts are also a possibility. One study comparing two consecutive emergences of the same brood, 17 years apart, found evidence of slight range expansion in some areas, though the changes were small and close to the limits of what the methods could detect.12Annals of the Entomological Society of America. Documenting Single-Generation Range Shifts of Periodical Cicada Brood VI (Hemiptera: Cicadidae: Magicicada spp.) Periodical cicadas spread very slowly because females lay eggs close to where they emerged, so any range expansion happens at a pace of tens of meters per generation. That sluggish dispersal rate creates a real vulnerability: if suitable habitat shifts faster than the cicadas can follow, populations can become stranded.
Broods That Are Shrinking or Already Gone
Periodical cicadas are organized into broods, each designated by a Roman numeral, that emerge in different years across different parts of the eastern United States. Not all broods are thriving. Brood VII, historically found in central New York’s Finger Lakes region, has contracted dramatically. By its 2001 emergence, the brood had vanished from counties where it was documented in earlier decades. The majority of the remaining population was confined to a small area within Onondaga Nation Territory. The decline occurred despite an increase in total forested area across the region, suggesting that it was not lack of trees per se but the historical fragmentation and destruction of the specific forests where cicadas had been living.13Journal of the New York Entomological Society. Decrease in Geographic Range of the Finger Lakes Brood (Brood VII) of the Periodical Cicada (Hemiptera: Cicadidae: Magicicada spp.)
The root cause across many declining populations is the same: historical forest loss, particularly the clearing of old-growth forests in the 19th and early 20th centuries. Research tracing the fate of broods in Iowa found that when old trees were removed, the nymphs feeding on their roots lost their food source and died. Even when forests regrew, the cicadas could not recolonize because they spread only about 50 yards per generation on average. Forests can regenerate in decades, but a periodical cicada population that has been extirpated from an area might take centuries to return, if it ever does.14Ecology. Periodical cicadas may be doomed to extinction
The story is not uniformly bleak, though. A multi-generational study in southwestern Ohio found that while cicada density declined in old-growth forest and rural woodlots between emergences, treed fencerows, which are often considered disturbed habitats, actually saw a doubling in density. The finding challenges the assumption that human-modified landscapes are always bad for periodical cicadas and suggests that certain corridor-like habitats with mature trees may serve as unexpected refuges.15Ecosphere. Multigenerational emergence trends of Brood X Magicicada spp. across habitat types in southwestern Ohio
What Lives Inside Them Underground
Spending 17 years feeding on nutritionally sparse xylem fluid raises a question about whether cicada nymphs get help from microbes. Research examining the gut microbiomes of nymphs from multiple 17-year broods found that different species of Magicicada from different broods and different geographic locations shared a core set of gut bacteria. Nymphs of M. septendecim from Broods II and VI and M. cassini from Brood X, species that are morphologically distinct and geographically separated, harbored similar microbial communities.16PubMed Central. Gut microbiome insights from 16S rRNA analysis of 17-year periodical cicadas (Hemiptera: Magicicada spp.) Broods II, VI, and X The consistency of this core microbiome across species and broods hints that these bacteria may play a functional role in helping nymphs extract enough nutrition from xylem over their extraordinarily long juvenile period, though the precise metabolic contributions remain an active area of research.